In order to enhance performances of ABS used in the vehicle, it is effective to conduct the control of lock-unlock at a state of a large friction coefficient on road surface as far as possible. The friction coefficient on road surface is dependent upon a slippage ratio of a wheel at a constant road surface state, so that ABS is designed so as to control the lock-unlock in the vicinity of the slippage ratio giving a maximum friction coefficient on road surface.
In the conventional ABS, it is general to use a system that the slippage ratio is determined by calculating from a speed of the vehicle and a rotating speed of the wheel measured and the braking is automatically controlled so as to enter this slippage ration into a given range.
However, the method of controlling the slippage ratio to obtain an optimum friction coefficient on road surface is effective on a constant road surface, but there is a problem in the actual running that even if the slippage ratio is controlled to the given range, the optimum friction coefficient on road surface is not obtained because a relationship between the slippage ratio and the friction coefficient on road surface is largely dependent upon the road surface material, weather and the like. For this end, it is desirable that forces of the road surface acted upon a tire in a peripheral direction and a vertical direction are measured and a friction coefficient is directly determined from the measured forces and the braking is controlled so as to make the friction coefficient measured optimum. Therefore, there is proposed a method of directly measuring forces acted upon the tire as described, for example, in JP-A-10-506346.
According to this conventional method for measuring the forces, plural pairs of magnet pair comprising two magnets arranged at two standard points, which are different in the position in a radial direction on the same radius of a sidewall portion of the tire, are arranged so as to separate apart from each other around a center axis of the tire, and a magnetic sensor is disposed and fixed to a vehicle at a radially position corresponding to each of the standard points, and a timing of directly facing the standard points relatively displacing with the rotation of the tire to the magnetic sensors corresponding thereto is gotten as a timing of developing a peak of a magnetic flux detected by the magnetic sensor, and relative displacement between the standard point in the magnet pair and relative displacement of the standard point between the pair of the magnet pair are calculated from a time lag of the timing between these standard points, and strains of the tire in a peripheral direction and a vertical direction are calculated based on these relative displacements, and forces acting to the peripheral direction and vertical direction are determined from the calculated strains and the known tire rigidity.
However, this method is required to calculate the relative displacement from the time lag by taking data of the rotating speed of the wheel changing at any time, so that there are problems that the control becomes complicated and the precision of the calculation is deteriorated by the influence of the precision on the rotating speed of the wheel.
The invention is made in the light of the above problems, and an object thereof is to provide a method and an apparatus for measuring forces acted upon a tire in which forces acted upon the tire in radial direction and peripheral direction of the tire required for the high precision measurement of friction coefficient on road surface can be measured simply in a high precision.